Question

A pea plant with purple flowers was crossed with white flowers, producing 50 plants with only purple flowers. On selfing, these plants produced 482 plants with purple flowers and 162 with white flowers. What genetic mechanism accounts for these results? Explain.

Answer 1

• A pea plant with purple flowers (dominant trait) was crossed with one having white flowers (recessive trait). Since all 50 offspring plants had purple flowers, this indicates that the purple-flowered parent was homozygous dominant or heterozygous, and the white-flowered parent was homozygous recessive. The F₁ generation showed only the dominant purple phenotype.
• Upon selfing (self-pollinating) these purple-flowered F₁ plants, the resulting F₂ generation showed 482 with purple flowers and 162 with white flowers, close to the expected 3:1 phenotypic ratio typical of monohybrid crosses with complete dominance.
• This happens because the purple flower color (dominant allele) masks the expression of the recessive white allele in heterozygotes. During gamete formation in the F₁ heterozygotes, the alleles segregate so that half the gametes carry the dominant allele and half carry the recessive allele. When fertilization occurs, the genotypic ratio in F₂ is approximately 1 homozygous dominant : 2 heterozygous dominant : 1 homozygous recessive, producing the 3:1 phenotypic ratio where three show purple flowers and one shows white flowers.

This mechanism is a classic example of Mendel’s Law of Dominance and Law of Segregation. The dominant purple flower allele expresses in the F₁, while the recessive white allele reappears in a fraction of the F₂ due to segregation of alleles during gamete formation and random fertilization.

Hence, this experiment is explained by a monohybrid cross with complete dominance of purple flower color over white, following Mendel's laws of inheritance.